A film scanner is an apparatus for obtaining an image printed on a film as digital data.
The arrangement of the film scanner will be explained with reference to FIG. 9.
FIG. 9 is a block diagram showing a scanner. Reference numeral 101 denotes a light source which irradiates a film 102. The film 102 is a positive color film or negative color film. Reference numeral 103 denotes an imaging lens; and 104, a CCD on which an image on the film 102 is formed via the lens 103. The CCD includes line sensor CDs for three colors, to which R, G, and B color separation filters are respectively adhered. An image on a film is color separated and output as analog signals. Reference numeral 105 denotes an A/D converter which converts an analog signal into a digital signal. The digital signal is output as 10-bit data for each of R, G, and B. Reference numeral 106 denotes a color table storage which stores color conversion tables for respective negative film types. The color conversion table is a one-dimensional LUT (Look-Up Table) for each of R, G, and B. That is, three LUTs are prepared for each film. FIG. 10 shows an example of the LUT. With this conversion table, 10-bit negative data is converted into 8-bit positive data. For example, as for color negative films, the color balance and tone characteristic change between films of respective types. Thus, an LUT must be prepared for each film. Reference numeral 107 denotes an image data converter which converts image data by using a one-dimensional LUT stored in the color table storage 106. A negative image is converted into a positive image with an adjusted color balance, resulting in 8-bit data for each of R, G, and B. Reference numeral 108 denotes a memory in which image data processed by the image data converter 107 is written. After an image of one frame is written, the image is transferred to a host PC (not shown).
A conventional scanning sequence executed under the control of the host PC will be explained.
<Step S1>
The user loads a film into a scanner.
<Step S2>
The scanner automatically determines a film type from a bar code printed on the film.
<Step S3>
A color table (LUT: Look-Up Table) corresponding to the film type is automatically set in the scanner. Alternatively, the user sets an LUT by designating an arbitrary film type.
<Step S4>
Prescanning for obtaining a low-resolution confirmation image is performed.
<Step S5>
The user selects an image to be actually used from prescanned images, and executes scanning for obtaining a high-resolution image.
In scanning a transparent original such as a photographic film, the original is irradiated by the light source. Transmitted light is converted into an electrical signal by the photoelectric converter, and the electrical signal is A/D-converted, obtaining desired digital data. The obtained digital signal is converted into a digital signal as a scanner output signal via a digital signal conversion table (look-up table).
In general, image scanning of this type is performed in two, prescanning and actual scanning.
Prescanning is first done at a low resolution. Prescanning conditions are unique settings determined depending on the film type such as a negative or positive film. A prescanned image is displayed as a preview image on the monitor. The user observes this preview image, manually sets the brightness, color balance, and the like for actual scanning, and then performs actual scanning.
According to this method, however, the user must determine actual scanning settings for each original, a long time is taken for processing many images, and a heavy burden is put on the user. To solve this problem, there is considered a method of analyzing a prescanned image by a computer, automatically setting scanning conditions for actual scanning on the basis of the analysis result, and performing actual scanning.
For example, the average brightness of an original is calculated from a prescanned image, the light quantity of the light source is changed in accordance with the brightness, and the CCD arrival light quantity is adjusted. Alternatively, the scanning speed (feed speed) is adjusted, and scanning suitable for each original is performed, effectively acquiring information printed on the original. In addition to this method, there is proposed another scanning method of executing pre-prescanning before prescanning.
That is, the prescanning precision is further increased in advance by adding one scanning step before prescanning so as to execute prescanning and actual scanning under conditions optimal for each film type with respect to various negative-base color film densities. As a result, the precision of subsequent actual scanning is increased. In this case, pre-prescanning is performed at unique settings determined in advance, acquiring a negative-base digital count in the negative-base region. Prescanning is executed at analog settings which change the negative-base digital count to a predetermined value. This increases the prescanning and actual scanning precisions.
An original is irradiated by the light source, transmitted light is converted into a linear electrical signal by the photoelectric converter at the original transmission ratio, and the electrical signal is A/D-converted into a digital signal. The obtained digital signal is further converted into a digital signal as a scanner output via a look-up table. By changing the look-up table, the scanner can cope with the film type such as a negative or positive film. For example, actually available scanners adopt a method of manually or automatically selecting a plurality of look-up tables for respective films that are accumulated in a host computer, and removing the color balance characteristic which changes between films of respective types.
In the above-described prior art, the film type is decided in film loading. In other words, an LUT for use is decided in film loading. The film type automatically selected in film loading or the film type arbitrarily selected by the user cannot be changed after prescanning, which poses a problem. The scanner may mistake automatic determination of the film type and misidentify the film type. In this case, if the user is not aware of the misidentification and performs prescanning, an LUT not suitable for the film is used, and a prescanned image is poor in color balance. At this time, the user becomes aware of misidentification of the film type, but cannot change the film type (LUT) and continue work because the film type and a corresponding LUT have already been decided. The user must interrupt work and restart it from film loading.
In the method of calculating the average brightness of an original from a prescanned image and deciding analog settings (light quantity, scanning speed, electrical gain, offset, and the like) for actual scanning, if prescanning is done using a look-up table which changes depending on the film type, the average brightness range changes for each exposure condition (underexposure, correct exposure, or overexposure) because the shapes of respective look-up tables are different. The exposure condition of the negative film of an original cannot be accurately grasped, and actual scanning cannot be performed at optimal analog settings.
In the method of determining an exposure condition from the average brightness of a prescanned image, if acquisition of the digital count of the negative-base region in pre-prescanning fails due to any reason, analog settings which set the negative-base digital count to a predetermined value cannot be obtained. The exposure condition cannot be determined from the average brightness calculated from the prescanned image.
A prescanned image is displayed as a preview image on the monitor. The preview image must be an image which simulates an image in actual scanning. When analog settings are the same between prescanning and actual scanning, the same A/D-converted digital data are input to the look-up table. A look-up table for a prescanned image is so changed as to obtain the optimal brightness and color balance, the prescanned image is corrected, and an actual scanning look-up table corresponding to the corrected prescanned image is set.
However, when analog settings are different between prescanning and actual scanning, different A/D-converted digital data are input to the look-up table. The colors of a preview image and actually scanned image cannot coincide with each other unless A/D-converted digital data in actual scanning is predicted from a prescanned image.
If a prescanned image is scanned using a look-up table for each film type in predicting A/D-converted digital data in actual scanning from a prescanned image, A/D-converted digital data in actual scanning cannot be predicted directly from the prescanned image because the look-up table for each film type is a conversion table which has a nonlinear relationship with A/D-converted digital data. In this case, the look-up table used in prescanning must be searched for on the basis of the digital count of the prescanned image, and A/D-converted digital data in prescanning must be calculated to predict digital data in actual scanning. This greatly complicates processing of making the colors of a preview image and actually scanned image coincide with each other.